Make Delayed::Worker.new idempotent

Calling Delayed::Worker.new multiple times would continue to re-add the
plugins to the lifecycle.

Some gems and projects use the pattern `Delayed::Worker.new.work_off` to
trigger delayed jobs in specs. Over time, the callback chain would get
very long and, in our case, caused a StackLevelTooDeep error.

Better would be to have a proper singleton: 

`Delayed::Worker.worker.work_off`

In the absence of a proper singleton, this will make the pattern work.

lib/delayed/worker.rb

@@ -25,7 +25,7 @@ class Worker # rubocop:disable ClassLength
     # Named queue into which jobs are enqueued by default
     cattr_accessor :default_queue_name
 
-    cattr_reader :backend
+    cattr_reader :backend, :lifecycle
 
     # name_prefix is ignored if name is set directly
     attr_accessor :name_prefix
@@ -39,6 +39,7 @@ def self.reset
       self.delay_jobs        = DEFAULT_DELAY_JOBS
       self.queues            = DEFAULT_QUEUES
       self.read_ahead        = DEFAULT_READ_AHEAD
+      @lifecycle             = nil
     end
 
     reset
@@ -97,7 +98,15 @@ def self.after_fork
     end
 
     def self.lifecycle
-      @lifecycle ||= Delayed::Lifecycle.new
+      # In case a worker has not been set up, job enqueueing needs a lifecycle.
+      setup_lifecycle unless @lifecycle
+
+      @lifecycle
+    end
+
+    def self.setup_lifecycle
+      @lifecycle = Delayed::Lifecycle.new
+      plugins.each { |klass| klass.new }
     end
 
     def self.reload_app?
@@ -112,7 +121,9 @@ def initialize(options = {})
         self.class.send("#{option}=", options[option]) if options.key?(option)
       end
 
-      plugins.each { |klass| klass.new }
+      # Reset lifecycle on the offhand chance that something lazily
+      # triggered its creation before all plugins had been registered.
+      self.class.setup_lifecycle
     end
 
     # Every worker has a unique name which by default is the pid of the process. There are some

spec/worker_spec.rb

@@ -154,4 +154,22 @@
       @worker.say(@text, Delayed::Worker.default_log_level)
     end
   end
+
+  describe 'plugin registration' do
+    it 'does not double-register plugins on worker instantiation' do
+      performances = 0
+      plugin = Class.new(Delayed::Plugin) do
+        callbacks do |lifecycle|
+          lifecycle.before(:enqueue) { performances += 1 }
+        end
+      end
+      Delayed::Worker.plugins << plugin
+
+      Delayed::Worker.new
+      Delayed::Worker.new
+      Delayed::Worker.lifecycle.run_callbacks(:enqueue, nil) {}
+
+      expect(performances).to eq(1)
+    end
+  end
 end